One of the common arrangements for adjusting the height of the seat of a chair is to mount the seat on a threaded spindle that engages a nut that is rotatably fixed with respect to the base of the chair when no weight is present on the seat. Rotation of the seat in either direction then raises or lowers it according to the direction of rotation. Placement of weight on the seat disengages a clutch mechanism that leaves the nut free to turn with the spindle as the occupant of the seat turns to face in different directions.
These mechanisms have tended to be somewhat difficult and expensive to manufacture. They can best be functionally compared on the basis of (a) the vertical distance between the bearings supporting the spindle, (b) the closeness of the upper bearing to the seat of the chair, (c) the least extent of the "stick-out" of the spindle at the downward extent of the adjustment for a given degree of adjustability and bearing spacing, and (d) the total amount of vertical adjustment or "stroke" allowed within comparable envelopes. The vertical distance between the bearings supporting the spindle against lateral forces has a significant effect on the wear characteristics of the mechanism, and the closeness of the upper bearing to the seat has an effect on the leverage of the lateral forces as they are applied. The present invention provides a simplified structure with regard to the cost of component parts and the assembly procedures, while retaining a combination of the desirable features listed above.